Glycosides are one of components widely distributed in botanical world, and are decomposed into sugar moieties and non-sugar moieties with a suitable acid, alkali or hydrolytic enzyme.
Generally, a non-sugar moiety is called as an aglycone and, particularly, in the case that an aglycone is a steroid, it is called as a genin. Depending upon chemical structures of non-sugar moieties, glycosides are divided into alkyl glycosides, phenol glycosides, cyanate glycosides, mustard oil glycosides, coumarin or oxycoumarin glycosides, flavonoid glycosides, anthraquinone glycosides and the like. Further, glycosides are also classified according to their physical and chemical properties such as saponins, cardiotonic glycosides, bitter glycoside and the like.
Regarding the significance of the existence of a glycoside in a plant, there can not be found any established theory and it is said that a plant converts a noxious substance in its metabolites into a glycoside to render the substance innocuous, or a glycoside is a stored substance.
Usually, a non-sugar moiety is attached to a sugar moiety through ether bond but, in a mustard oil glycoside, it is attached to a sugar moiety through thioglycoside bond.
When a sugar moiety is glucose, such a glycoside is called as a glucoside. Although there can be two kinds of glucosides, namely .alpha.- and .beta.-glucosides , .beta.- glucoside is predominant, which is hydrolyzed by .beta.-glucosidase. Examples of a sugar moiety include hexoses such as glucose, fructose, mannose, galactose, etc.; and pentoses such as xylose, arabinose, etc. Further, according to the number of sugars, monosaccharides, disaccharides, trisaccharides and the like are included.
Examples of a non-sugar moiety called aglycone or genin includes alkaloids, steroids, saponins, flavonoids and the like and many of them have remarkable physiological activities against animals. Therefore, they are utilized as important substances in medicines, toxicants, perfumes, seasonings, pigments and the like.
Usually, these useful substances are extracted and separated with organic solvents, aqueous solvents and the like and then subjected to a treatment suitable for a particular purpose, for example, concentration, fractionation, purification and the like.
In this regard, a glycoside is extracted and separated from a plant or other suitable starting material in its intact form, or a free aglycone is extracted by hydrolyzing a glycoside in the starting material into a sugar moiety and a non-sugar moiety during treatment thereof. That is, usually, extraction and separation of a glycoside with an organic solvent or an aqueous solvent is carried out with paying attention to hydrolysis of the glycoside to obtain the glycoside in its intact form, or without paying any special attention to hydrolysis. For obtaining a glycoside in its intact form, it is of importance to inhibit the action of a hydrolytic enzyme present in a plant or other starting material and it is necessary to pay attention to secondary changes during operation. pH and temperature conditions are also of importance.
When extraction and separation are carried out without paying any special attention to hydrolysis, usually, an aglycone is liable to be affected by a certain factor except that the aglycone is stable against an extraction solvent, extraction conditions, coexisting substances and the like.
Recently, an attempt has been made to extract and separate a glycoside from a plant by using a fluid at supercritical conditions, particularly, carbon dioxide at supercritical conditions as an extraction agent. However, it is difficult to extract saccharides and glycosides having many polar groups with carbon dioxide alone under such pressure conditions as at 200 to 300 kg/cm.sup.2 which is considered to be economical. In view of this, it has been reported to carry out extraction with addition of water, ethanol or the like as an extraction auxiliary (entrainer) to impart polarity to carbon dioxide extraction agent. However, it is still inefficient.
Since a sugar moiety of a glycoside is not specifically utilized as an objective substance having physiologically activities as described above and the objective substance to be extracted must be its aglycone, it is considered that a desired result can be achieved only by extracting and separating an aglycone without any impairment in its quality.
In a conventional method, extraction of an aglycone is carried out by allowing to stand a cut or pulverized starting material of a plant under humidified and warmed conditions for a long period of time to activate a hydrolytic enzyme contained in the material itself to hydrolyze a glycoside into a sugar moiety and a non-sugar moiety, or by subjecting a cut or pulverized starting material to hydrolysis in an organic solvent or an aqueous solvent for a long period of time with adjusting pH at an elevated temperature. In this case, an aglycone is liable to impair its quality because extraction takes a long period of time and it is affected by pH and temperature conditions, coexisting substances and the like. Further, when the aglycone is separated from an extraction solvent, there is such a defect that a light fraction is liable to scattered and lost.